Automatic toll switching telephone system



June 30, 1942.

P. B. MURPHY AUTOMATIC TOLL SWITCHING TELEPHONE SYSTEM Filed Dec. 3l, 1940 l0 Sheets-Shea?l l v. .Ofi

WE/v ro@ P. B. M URPH Y A TTORNEV June 30, 1942. P, B, MURPHY 2,288,251

AUTOMATIC TOLL swITCHING TELEPHONE SYS-TEM Filed Dec. 31, 1940 l0 Sheets-Sheet 2 MA RG/NL A 7" TOR/VE V 1m 3o, 1942. P. B. MURPHY 2,288,251

/ AUTOMA'C TOLL SWITCHING TELEPHONE SYSTEM Filed Deo. 31, 1940 10 Sheets-Sheet 3 KEY MON/TOR P. B. MURPHY June so, 1942. y

AUTOMATIC TOLL SWITCHING TELEPHONE SYSTEM vFiled Dec. 31, 1940 l0 Sheets-Sheet 4 /NVENTOR Re. MURPHY ATTORNEY June 30, 1942. P. B. MURPHY 2,288,251

AUTOMATIC TOLL SWITCHINC- TELEPHONEy SYSTEM FJ' led Dec. 3l, 1940 1Q Sheets-Sheet 5 r v v v l v v v v /NVEn/TOR u? 5 RB. MURPHY U A TTOR/VEV Jane 3o, 1942. R E, MURPHY 2,288,251

y AUTOMATIC TOLL SWITCHING TELEPHONE SYSTEM Filed Dec. 31, 1940 l0 Sheets-Sheet 6 ATTORNEY June 30, 1942. P, B, MURPHY 2,288925.

AUTOMATIC TOLL SWITCHING- TELEPHONE SYSTEM Filed Deo. 3l, 194C 10 Sheets-Sheet 7 .FENDER A T Tom/5V June 30, .1942.

P. B. MURPHY AUTOMATIC TOLL SWITCHING TELEPHONE SYSEM Filed Dec. 31, 1940 10 Sheets-Sheet 8 /A/l/E/VTOR P. 5. MURPHY A TTORNEV June 30, V1942. P B MURHY 2,288,251

AUTOMATIC TOLL SWITCHING TELEPHONE SYSTEM Filed Deo. 51, 1940 10 Sheets-Sheet 9 /N VEN TOR R B. MURPHY ATTORNEY June 30, 1942- P`. B. MURPHY AUTOMATIC TOLL SWITCII'ING TELEPHONE SYSTEM Filed Dec. 3l, 1940 10 Sheets-Sheet l0 INI/EN TOR P. B. MURPHY Patented June 30, 1942 usarsi AUTOMATIC TOLL S WITCHING TELEPHONE STEM Paul B. Murphy, South Nyack, N. Y., assigner to Bell Telephone Laboratories, Incorporated, New York, N. Y., a corporation of New York Application December 31, lflil, Serial No. 372,560

16 Claims.

This invention relates to telephone exchange systems and, more particularly, to systems in which connections are established between subscribers lines terminating in diiferent offices over toll trunks by dial switching mechanism under the control of a toll operator.

In the establishment of connections to oiiices of a dial switching area for which toll rates should be assessed'for service, it is customary for a calling subscriber in such an office who desires a connection with another subscriber at a remote point to dial a number whereby a connection is rst obtained with a toll operator who then proceeds to set up the desired connection and to supervise the assessment of the required toll charge.

Several methods of establishing' such toll connections have heretofore been proposed for use in connection with dial switching systems. In

accordance with one of these methods, disclosed by way of example in Patent No. 1,780,906, granted November l1, 1930, to W. W. Carpenter and R. E. Hersey, a toll connection incoming from a distant toll point is answered by the toll operatoi` who then terminates the call by plugging directly into an idle trunk extending to the dial switching oflice in which the desired subscribers line terminates which trunk is then `automatically associated over a link circuit with an idle register sender, and by then keying the directory numfil ber of the desired line on a key-set associated with her position cord circuits to control the registration of the line number in the seized idle' sender. The registration is controlled by the keyset through the transmission of marginal` and I.

reverse current impulses over the talking conductors of the cord circuit and trunk, through the link circuit to the responsive equipment in the sender. The sender after having registered the line designation then proceeds tc control the setting of dial switching apparatus in accordance therewith to extend the connection from the trunk to the desired line.

In some cases it is desirable tc include in an exchange area offices which are located on the fringe of such area and to control the extension of toll connections tc lines terminating in such oiices from toll operators positions located in the main exchange area; Under these conditions the trunks extending from the toll cperators position to the fringe oflices might be of considerable length and since the register sender equipment would be located in the terminating ofiice, the transmission of key-pulsing impulses over such a long trunk for setting the digit registers of a selected register sender by the method disclosed in the Carpenter et al patent hereinbefore referred tc might prove to be unreliable due to the losses incident to the transmission of direct current impulses over long cable paths and to the fact that certain digit codes involve the transmission of impulses of different voltages requiring the response of sensitive and marginal responsive relays in the sender.

It is therefore an object of the present invention to provide apparatus whereby a toll operator may transmit impulses from her key-set over a long trunk tc control the registration of a desired line number in the sender of a terminating cliice in an emcient and reliable manner.

It is a further object of the invention to enable key pulsing to be accomplished by the transmission and reception of codes of impulses comprising combinations of a plurality cf frequencies.

IThe present invention has been illustrated as applicable to a system in which the terminating dial office to which a toll connection may be established is equipped with a call distributing B cperators position, link circuit and sender equipment for terminating incoming calls from toll or manual offices, of the type disclosed in Patent 2,020,458, granted November 12, 1935, to J. Dahl. The toll completing trunks incoming from the toll cperators position in the rmain exchange area terminate in the terminating ofiice in incoming selector switches and are ccnnectaole over link circuits of the terminating omce with a key impulse receiving circuit and with a register sender.

The toll operators position circuit is provided with a ten-button key-set common to the cord circuits of her position which becomes associated with a cord circuit when she takes it for use and equipment is provided common to all of the operators positions for generating a plurality of different alternating current frequencies, for combinding such frequencies in different combinations of two frequencies each and applying them to the keys of the key-set, to the contacts of a key-pulsing signaling relay and to the contacts of a start relay also provided at each operators position. As illustrated, the six frequencies employed are generated by vacuum tube oscillators of the type disclosed in the application of L. A. Meacham, Serial No. 372,557, filed December 31, 1%0, but such frequencies might be generated by other well-known types of generators. The frequency generating equipment is provided in duplicate and automatic alarm and transfer equipment is provided for transferring the connections between the operators positions from one generating equipment to the other in the event any frequency generator fails.

The key-pulsing receiving equipment at the terminating office is, as previously stated, associable with a calling incoming trunk and with a call distributing terminating sender by a link circuit in the same manner as the call distributing B operators position and key-set circuit of the terminating office are associated. The conductors over which the alternating current key-pulsing signals are incoming to the receiving equipment are connected through transformers and amplifying tubes of the pentode type to six receiving channels each of which chan-nels includes a lter circuit, a space discharge responsive device and a signal responsive relay, the different channels being responsive respectively to signals of the frequencies generated by the six frequency generators associated with the operators position circuits. The receiving circuit is also provided with translating equipment which is eifective in response to the selective operation of the signal responsive relays of the receiving channels in response to keyed digits, to make such translations of the incoming signals as are necessary to set the digit registers of the terminating sender at the time associated with the receiving equipment to register the keyed digits. The receiving equipment is also provided with a volume limiter, circuits for enabling the space discharge devices of the receiving channels and circuits for permitting the transmission of signals by the signal responsive relays of the receiving channels to the translating equipment only after a key-pulsing signal has been received and if no signal within the voice frequency range is present upon the termination of the key-pulsing signal thereby eliminating the possibility of false registrations which might result due to the presence of conversational current on the signaling circuit prior to key pulsing.

In general the system disclosed operates as follows: In response to an incoming toll call which is to be terminated in the dial switching office disclosed, the operator plugs into an idle trunk extending to such oflice whereupon a start circuit is closed which causes the trunk finder of an idle link circuit to hunt for the trunk, which causes the position nder of the link circuit to hunt for and select an idle key-pulsing receiving circuit and which causes the sender finder of the link circuit to hunt for an idle sender. The operator then operates a key-pulsing key at her key-set circuit and as soon as a signal is received from the terminating onice that a sender has been selected a relay is operated to transmit a preliminary key-pulsing signal comprising two of the frequencies generated by the frequency generating oscillators. A predetermined interval following the receipt of this key-pulsing signal the enabler circuit functions to render the space discharge devices operable to receive the signal frequencies whereupon if only the two frequencies transmitted for the signal are received and therefore only the corresponding responsive relays are operated, relays in the receiving circuit are operated to establish operating circuits for translating relays controllable by the responsive relays in response to subsequently keyed digits. If, however, frequencies witliin the voice frequency range should be present in addition to the signaling frequencies, the receiving circuit becomes blocked until such extraneous frequencies disappear.

Following the transmission of the key-pulsing signal the operator depresses keys of her key-set to transmit signals in accordance with the digits of the wanted line number, the enabling circuit responding as before to render the space discharge devices responsive a predetermined interval following the reception of each signal but at this time disabling the space discharge devices after a further interval following the reception of each signal. The responsive relays selectively operated in response to the transmission of each digit signal are eifective to selectively operate the translator relays to set the registers of the sender '1n the same manner as though the digits had been keyed on a key-set of the call distributing B operators position. Following the completion of keying the operator depresses a start key at her position which is effective totransmit a distinctive start signal to the receiving circuit for signaling the sender that all digits have been keyed and that therefore the sender may proceed to set up the connection in accordance with the digital information registered therein.

The invention having been considered in a general manner, reference may now be had for a clearer understanding thereof to the following detailed description thereof taken in connection with the accompanying drawings in which:

Figs. 1 and 2 taken together disclose in the upper portion of Fig. 2 one of the cord circuits of an operators position, in the lower portion of Fig. 2 the position circuit common to all cord circuits of the position and in Fig. 1 the grouping and operators telephone circuit;

Fig. 3 shows the key-set circuit of the operators position of Fig. 2;

Figs. 4, 5 and 6 taken together show the signaling frequency generators, switching and alarm circuits common to a plurality of operators positions, Fig. 4 disclosing the vacuum tube oscillators for generating six signaling frequencies and the transformers by means of which these frequencies are grouped in the proper combinations for signaling purposes; Fig. 5 showing switching circuits for connecting the oscillators of Fig. 4 and a second set of oscillators indicated by the box at the right of Fig. 5 to the key-set circuits such as disclosed in Fig. 3 of operators positions; and Fig. 6- showing alarm equipment and equipment operable to control the switching circuits of Fig. 5 upon failure of any oscillator tube;

Fig. 7 shows a trunk outgoing from the toll operators position and incoming at a distant terminating oiiice in an incoming selector switch indicated by the box in the upper right-hand corner thereof, a link circuit represented by the box so labeled, a sender represented by the box so labeled and a portion of the apparatus of a key impulse translator circuit;

Fig. 8 shows the remaining portion of the translator circuit;

Figs. 9 and 10 taken together disclose the key impulsing receiving circuit, Fig. 9 showing the tuned frequency channels each comprising a filter circuit, a gas-filled tube and a signal responsive relay, blocking and switching relays and a portion of the channel enabling circuit, and Fig. 10 showing the amplier tubes, the volume limiter and the remaining portion of the enabling circuit; and

Fig. 11 is a chart showing how the several sheets of the drawings should be assembled to disclose the complete invention.

As previously stated, the incoming selector, the trunk and sender represented by the boxes in Fig. 7 may be of the types disclosed in Patent No. 2,020,458 to J. F. Dahl hereinbefore referred to and the nal selectors employed in completing connections to desired lines in the terminating oice may be of the well-known panel type.

It will be assumed that a toll call is incoming to the operators position of Figs. 1 and 2 and that the operator answers the call on the toll lines by plugging the plug 200 of a cord circuit of her position into the toll line jack 20|. She then depresses her cord key 202 to the right or -talking position. A circuit is thereupon established for relay 203 extending from battery, through the winding of relay 203, the alternate contacts of key 202, conductor 204 to ground at the inner lower normal contacts of relay of the operators telephone circuit. Relay 203 upon operating closes a circuit for relay |00 extending as traced from ground on conductor 204 over the alternate contacts of key 202, the No. 1 lower front contacts of relay 203, conductor 205 to battery through the winding of relay |00. Relay |00 thereupon operates removing ground from conductor 204 to prevent another cord circuit relay, such as relay 203, from operating. With relay |0| operated, since the operators telephone set |02 will have been plugged into the position jack to establish the circuit thereof extending from battery through its winding, the upper contacts of jack |03 to ground at the cuter right back contact of relay |04, relay |00 closes a holding circuit for cord circuit relay 203 which may be traced from ground over its inner lower alternate contacts, the No. l upper front contacts of relay |0|, conductor |05, the No. l upper front contacts of relay 203, the upper alternate contacts of key 202 to battery through the winding of relay 203. Thereby relay 203 individual to the cord circuit and relay |00, common to all cord circuits of the position, are held operated. Relay 203 splits the cord circuit and associates conductors 20E to 203 with the cord circuit.

The operation of relay |00 connects the tip and ring conductors |00 and |01 of the operators headset circuit over its outer iront contacts and the No. 2 upper an`d No. l lower front contacts of relay |0i to conductors |08 and |00 extending to the leit or monitoring contacts of the key 202 and at its inner upper back contact disconnects the winding of relay l0 from conductor circuit of the operators position is operated to the left or monitoring position the operator will be able to talk or listen on both cord circuits but will be able to split, transfer or key pulse on only the cord circuit the talking key of which has been operated to the right or talking position. Upon the operation of relay 203 a circuit is also established from high resistance ground on the sleeve of jack 20i over the sleeve of plug 200, the No. 2 lower contacts of relay 20,3 to battery7 through the windings of relays 2i0 and 2l|. Relay 2|| upon operating closes an obvious circuit for relay 2i2 but relay 2li! being marginal does not operate at this time. At its inner upper ront Contact relay 2|2 connects ground through high resistance 2 3 and over the No. 3 lower conftacts of relay 203 to the rear cord supervisory lamp 2|4 to warm the lamp without lighting it thereby improving its ashing characteristic. At its lower back contact relay 2l2 disconnects the tip of the answering plug 200 from the busy test conductor 2|5 which is connected over the No. 4 lower front contact of relay l0| through the winding of relay ||2 to ground and through condenser H3 to ground through the primary winding of transformer H4 and thence inducively to the operators headset 02. At its outer front contacts relay 2|2 also establishes a talking circuit which may be traced fro-rn the tip of the plug 200 over the No. 5 upper contacts of relay 223, conductor 203, the No. 2 upper back contact of relay 300, conductor 305, the lower right normal contacts of splitting key 2te, the lower front contact of relay 2|2, the lower back contact of relay 2|1, conductor H5, the lower No. 5 con- I If at this time the key 202 of `another cord e tacts of relay |0|, conductor |01, through the lower windings of repeating coil H6, inductively coupled to the operators headset transmitter, and over the back contacts oi relay |'|0 to the operators headset receiver, thence returning over conductor |00, the No. 3 upper contacts of relay |01, conductor ||1, the upper back contact of relay 2|1, the upper front contact of relay 2 i2, the upper right normal contacts of splitting key 2 I6, conductor 302, the No. 3 lower back contact of relay 300, conductor 200, the No. 5 lower contacts of relay 203 to the ring of plug 200. The operator is now enabled to receive instructions over the toll trunk for the further extension of the connection.

Having received instructions and it having been assumed that a connection is desired with a subscribers line which terminates in a dial switching oice accessible over a group of trunks of which one is disclosed in Fig. '7, the operator proceeds to test such trunks by touching the tip of plug 224 to the sleeves of the trunk jacks. A circuit is thereupon established from the tip of plug 224 over the No. 4- upper contacts of relay 203, conductors 208, the No. 4 upper back contact of relay 303, conductor 304, the lower left normal contacts of splitting key 2|0, the lower back contact of relay 2|8, conductor 2|5, thence as previously traced to ground through the winding of relay ||2 and through the primary winding of transformer ||4. It will be assumed that the trunk terminating in jack tests idle and that the operator on receiving no busy click in her headset inserts the plug 224 into jack 100. A circuit is thereupon established from ground through the high and low resistance windings of sleeve relay 10| in series over the sleeves of jack 100 and Vplug 224, the No. 3 upper alternate contacts of relay 203 to battery through the windings of relays 2|0 and 220. Sleeve relay 10| operates in this circuit establishing an obvious circuit for relay 102 which operates establishing a direct current loop circuit over the two-conductor trunk extending to the terminating oiiice. This loop circuit may be traced from battery at the terminating oiiice through the lower winding of relay 100, the lower back contact of relay 109, the lower left winding of repeating coil 123, over interoiice trunk conductor 105, the lower right winding of repeating coil 104, the contacts of relay 102, the winding of relay 103, the upper right windingy of coil 104, trunk conductor 101, the upper left winding of repeating coil 123, the upper back contact of relay 14.0 and through the upper winding of relay 100 to ground. Relays 103 and 100 both operate, relay 100 closing an obvious circuit for relay 150, and relay 103 connecting direct ground through the upper or low resistance winding of relay 10| to the previously traced sleeve circuit thereby operating both relay 2|0 and relay 220 at the operators position circuit.

Relay upon operating establishes an operating circuit for relay 149 extending from battery through the winding thereof, over the back contact of relay 1513 to ground over the upper contacts of relay '|50 whereupon relay 140 operates to open the previously traced loop circuit through the windings of relays 103 and 100. Relay 100 does not release, however, being held operated in a local circuit through both of its windings and over the front Contact of relay 103. Relay 103, however, releases again removing low resistance ground from the trunk sleeve circuit thereby releasing marginal sleeve relay 2|9. Relay 2|9, therefore, operates only momentarily and without eiect at this time.

Relay 220 upon operating establishes an obvious circuit for relay 2|8 which at its upper iront contact connects ground through high resistance 22|, over the No. 2 upper contacts of relay 203 to the circuit of calling supervisory lamp 222 for warming such lamp without lighting it to improve its flashing characteristic and establishes a talking circuit from the tip conductor 109 of the outgoing trunk over the tips of jack 100 and plug 224, the No. 4 upper front contact of relay 203, conductor 208, the No. 4 upper back contact of relay 303, conductor 304, the lower left contacts of splitting key 2|6, the inner lower front contact of relay 2|8, the lower back contact oi relay 223, conductor I I5, thence as traced to the operators telephone circuit and returning over conductor H1, the upper back contact of relay 223, the inner upper front contact of relay 2I8, the upper left normal contacts of key 2|0, conductor 305, the No. 2 lower back contact of relay 303, conductor 201, the No. 4 lower front contact of relay 203, over the rings of plug 224 and jack 100 to the ring conductors 1|0 of the outgoing trunk. At its lower back contact relay 2|8 opens the busy test circuit previously traced. Relay 150 upon operating also closes a circuit extending from battery through the winding of relay thence over the lower contacts of relay 150 to ground over the normal or position |1 contacts of sequence switch cam 152 whereupon relay 15| operates, locks in a circuit from battery through its winding, over the lower contacts of relay 150, to ground over its own lower contacts and connects ground to the start circuit for starting a preselected idle link circuit, such as is represented by the box 108, and which is fully disclosed in the patent to J. F. Dahl hereinbefore referred to, whereupon the trunk inder 1| I of the link circuit is started in search of the calling trunk. As soon as the trunk is found the link circuit sequence switch is advanced thereby causing the advance of the sequence switch of the incoming selector into position I8 and the selector 1|2 of the link circuit is started in Search of an idle key-pulsing receiving circuit. An idle receiving circuit is identified b-y battery connected through resistance 800 over the lower back contact of relay 80|, conductor 802, the lower back contact of relay 1|5 to a test terminal 1I6 appearing in the bank of the selector 1I 2. When the selector 1|2 nds the first idle receiving circuit, assumed to be the circuit disclosed in Figs. '1 to 10, inclusive, a test relay in the link circuit operates to arrest the hunting movement of the selector and to advance the sequence switch of the link circuit. A circuit is thereupon established from battery through the lower winding of supervisory relay 1|1 in the trunk circuit, through the lower right winding of repeating coil 123, over the contacts of sequence switch cam 154, over the ring talking conductor extending through the link circuit, ring conductor 1|8 of the receiving circuit, resistance 1I9, both windings of relay 120, resistance 12|, tip conductor 122 of the receiving circuit, tip talking conductor extending through the link circuit, the contact of cam 155, upper right winding of repeating coil 123 to ground through the upper winding of relay 1I1.

Due to the inclusion of the upper high resistance winding of relay 120 in the circuit thus traced, marginal supervisory relay 1I1 does not operate but relay 120 operates and closes an obvious circuit for relay 1|5 which also operates.

Relay 1I5 upon operating opens at its lower back contact the connection from battery through resistance 800 over the test terminal set 1I6, thereby releasing the test relay in the link circuit to advance the sequence switch of the link circuit into the position for starting the sender selector 1|3 to hunt for an idle sender. Relay 1|5 also closes an obvious circuit for the busy lamp 808; connects ground to the off-normal conductor 124; establishes a circuit extending over conductor 125 through the lower winding of make-busy relay establishes a circuit for the peg-count register 126 which may be traced from battery through its Winding and the winding of relay 121, inner upper back contact of relay '|28 to ground at the upper front contact of relay 1|5 whereupon the register is operated and locked in its operated condition over its armature contact and the front contact of relay 121; and connects battery over its lower front contact, the lower back contacts of relay 128 and through the resistances 129 and 130 to conductors 13| and 132 extending through the link circuit 108 toward the sender 1I4.

When the sender has been seized by the link circuit, battery thus connected to conductors 13| and 132 causes the sender to connect battery through an 840 ohm resistance to a conductor extending therefrom through the link circuit to conductor 133, thence through the winding of relay 134 to ground at the back contact of relay 135. Relay 134 thereupon operates locking over its upper front contact to ground on conductor and closing a circuit from ground on conductor 125 over its next to upper front contact through the winding of relay 135 to battery which relay thereupon operates, locks to ground on conductor 125 over its inner upper front contacts and closes an obvious circuit for relay 128 which also operates. Relay 128 upon operating opens the circuit of the peg-count register 126; disconnects battery from conductors 13| and 132 and connects battery over the lower front contact of relay 1I5 and its own lower front contacts to conductor 136 extending to the impulse receiving circuit of Figs. 9 and l0. With battery disconnected from conductors 13| and 132, the sender responds to disconnect battery from conductor 133 whereupon relay 134 releases, closes a shunt around the high resistance winding of relay 120 extending from the left terminal thereover over the lower back contact of relay 134 and the lower front contact of relay 135 to the right terminal thereof and at its inner lower back contact connects conductor 133 over the inner lower front contact of relay 135 to conductor 131. Conductors 13|, 132 and 133 are now extended from the sender 1|4 to the contacts of the translation relays 803 to 801, inclusive. y

Following the short-circuiting of the upper winding of relay 120, the current flowing through the windings of marginal relay 1|1 of the trunk is increased to such an extent that relay 1|1 operates to close the circuit of relay 153 which may be traced from battery through its winding and normal contacts, over the contacts of relay 1|1 to ground over the upper contacts of relay 150. Relay 153 locks over its alternate contacts to ground at the lower contacts of relay 15| and opens the circuit of relay 149 which releases to restore' the loop circuit through the winding of relay 103. Relay 103 now operates to reconnect low resistance ground to the sleeve of the trunk through the upper low resistance winding of sleeve relay 10|. In response to theY increased current now flowing over the sleeve circuit previously traced through the windings of relays 2'I0 and 220, marginal relay 2|0 now operates to connect ground over its contacts through low resistance 225, the No. 2 upper contacts of relay 203, to the supervisory lamp 222 whereupon such lamp lights as a signal to the operator that a sender has become associated with the extended connection.

Since the call is an inward call, the' operator may depress the front cord key-pulsing key 300 as soon as she has plugged up the cord whereupon a circuit is established from ground over the No. 3 upper back contact of relay 300, conductor 3| I, lower front contact of relay 2 I8, conductor 309, over the right contacts of key 305, thence in parallel through the winding of relay 301 and the non-inductive resistance 30S to battery and a circuit is also established from battery through the winding of relay 303, left contacts of key 306, conductor 300, lower contacts of relay 2I8, conductor 3| I, to lground at the No. 3 upper back contact of relay 300. Relays 301 and 303 both operate, relay 301 upon operating extending its operating ground over its lower contacts, the upper back contact of relay 3|4 and the upper front contacts of relay 301, to battery through the winding of relay 3i2. Relay 3I2 thereupon operates and locks itself from ground on conductor 300 over the No. 1 upper contacts of relay 303, over its own upper contacts, the normal contacts of relay 3|3 and through its own inner lower front contact and winding to battery and extends this locking circuit from its upper contacts through the No. 1 lower contacts and winding of relay 303 to battery. Relay 303 upon operating closes the circuit of lamp SI5 in a circuit through resistance 3I6 and over the No. 3 upper contacts of relay 303 and at its No. 4 lower contacts prepares the circuit for relay SIA. Relay 301 is now held operated under the control of relay 3|4 overa circuit extending from battery through its winding and outer lower contacts, over the upper back contact of relay 3M and the No. 1 upper contacts of relay 303 to ground on conductor 309.

Before proceeding with the operations incident to keying the wanted line number, the manner in which the signaling frequencies are generated and made available to the key-set circuits will rst be described. For each call the operator causes the transmission of a key-pulsing signal, may cause the transmission of a start signal and may transmit signals in accordance with any one of the digit values 1 to 0, inclusive.

These twelve possible signals comprise different combinations of two frequencies each from a total of six available frequencies. For generating these six frequencies two groups of Vacuum tube oscillators are provided, one group comprising six oscillator circuits 00| to 405, inclusive, for generating alternating current frequencies of '100, 900, 1100, 1300, 1500 and 1700 cycles, respectively, being disclosed in Fig. 4, and the other group, comprising six similar oscillator circuits, being represented by the box 500 in the right portion of Fig. 5. These oscillators are of the type fully disclosed in the application of L. A. Meacham, Serial No. 372,557, filed December 3l, 1940.

The alternating current output circuits of the oscillators 40| to 400, inclusive, are connected to conductors 4I| to M6, respectively, of cable 4|1 which conductors are connected through resistances and condensers to the control electrodes of gas-filled tubes 62| to 026, respectively, of the alarm and control circuit of Fig. 6. For example, conductor 4H is connected through resistance 00| and condenser SII to the control electrode or" tube 02|. These conductors are also multipled' as required to the terminals of the primary windings of transformers KP, 0, I, 2, 3, 4, 5, 5, 1, 8, 9 and ST disclosedin the right portion of Fig. 4 whereby each primary winding is bridged across the output circuits of two' of the oscillator circuits. For example the primary winding of transformer KP is connected at one terminal to conductor M6 and to the output circuit of the 1700' cycle oscillator circuit 005Y and at its other terminal to conductor @I0 and to the output circuit of the 1300 cycle oscillator circuit 400. The primary windings of other transformers are connected in accordance with the following table:

Table I Code signals Frequency f 0 l 2 3 4 5 6 7 8 9 KP ST x x x x x X X X X X X X X X x x x x X X X X The secondaries of these transformersare connected over the normal' contacts of switching relays 50| to 505, inclusive, topairs' of conductors extending in multiple to the key-set keys; t0 contacts of the key-pulsing relays such as 3M and to the contacts of the sta-rt relays' such as SIS of all odd numbered operators positions. Similarly the seconolariesV of the transformers ofthe oscillator circuits represented by the box 500 are connected over the normal contacts of switching relays 5| to 515, inclusive, to pairs'of conductors extending in multiple to the key-set keys, to contacts of the key-pulsing relays and to the contacts of the start relays of all evennumbered operators positions such asY the position illustrated. Thus: normally the oscillators of Fig. 4 supply codes of signaling frequencies to one half of the operators positions and? the oscillators represented by the box 500 supply codes" of signaling frequencies tothe other halfof the operators positions. If switching relays 50| to 505, inclusive, are operated the secondaries' of the transformers of Fig. 4- are disconnected at the back contacts of such" relays from the pairs of conductors mult-ipledl to the odd; numbered operators positions and such pairs' ofl conductors are connected over the front contacts ofsuch-relays to the corresponding pairs of conductors extending' to the even numbered positions whereby all positionsY are supplied with signaling frequencies from the oscillator circuits represented by the bex 500. on-the' ether'hnd if switching relaysr 5|| to` inclusive, are op-v erated, the secondarie's of the tran'sfo'rrner's of the oscillator circuits represented by the" box 500are disconnectedj at the back contacts of suchA relays from the pairs of conductors' multipled to the even numbered operators positions andl such pairs of' conducto-rs' are connected' over 'the front contacts'of such relays to the corresponding pairs ofconduct'ors extendin'gto odd numbered" positions whereby all `positions' are supplied with signaling frequencies from the oscillator circuits of Fig. 4.

Normally the gas-filled tubes 62| to 626, inclusive, are maintained in a conducting condition by the presence of signaling potential applied to their control electrodes over the conductors 4|| to 4|6, inclusive, in addition to the potential applied thereto from the potential divider extending from ground through resistance 6|1, resistance 6|8 and the positively poled battery 6|9 to ground. The associated relays 63| to 636, inclusive, are thus maintained operated, For example, relay 63| is maintained operated in a circuit from the positive potential junction 620 on the potential divided through resistance 6|0 across the cathode path through tube 63|, through the winding of relay 63| to ground through the upper left winding of transformer 621. Should the oscillator circuit 40| fail, the output potential applied therefrom over conductor 4|| to the control electrode of the tube 62| would cease thereby lowering the potential on the control electrode to such an extent that upon the next negative half wave of the alternating current source 628, the negative potential applied to the control electrode therefrom will so oppose the potential applied to the control electrode from the source 6|9 as to cause the tube to become extinguished thereby releasing the associated relay 63|. Relays 632 to 636, inclusive, become released in a similar manner upon the failure of the associated oscillator circuits 402 to 406, inclusive.

The release of any one of the relays 63| to 636, inclusive, establishes a circuit from ground over the front contacts of relay 629, the lower back contact of the released relay, for example, relay 63|, to battery through the winding of relay 601 whereupon relay 601 operates in turn operating the switching relays 50| to 505, inclusive, over conductors 506 and 501, thereby removing the group of oscillator circuits disclosed in Fig. 4 from service. At the same time, the release of any one of the relays 63| to 636, inclusive, establishes a circuit from ground over the front contacts of relay 629, the upper back contact of the released relay, for example relay 63| to battery through the associated lamp, for example lamp 64|, to indicate to the maintenance man which of the oscillator circuits has failed. Relay 661 upon operating also connects ground over the normal contacts of key 608 and its own lower front contacts to conductors 641 and 648 extending to visual and audible alarm devices.

The maintenance man noting the operation of the alarm devices then operates key 608 to stop the operation of such devices, to close a holding circuit for operated relay 601 and to light guard lamp 609. Noting the lighted one of the lamps 64| to 646, inclusive, for example lamp 64|, the maintenance man is thus apprized of the particular oscillator circuit which has failed. He may then take steps to rectify the trouble and may then listen to the output of the oscillator circuit to determine if the trouble has been corrected by plugging a headset receiver into the testing jacks associated with such oscillator circuit. `The testing jacks for the oscillator circuit 46| are disclosed at 401 and 408 and are associated with the output circuit of the oscillator circuit by the transformer 409. After the trouble has been fully corrected the gas-lled tube associated therewith will again function to operate the associated relay thereby extinguishing the associated lamp and opening the initial operating circuit of relay 601. The maintenance man may then release key 608 to release relays 601 and 50| to 505, inclusive, thereby restoring the oscillator circuits of Fig. 4 to service. Upon the release of key 608 the guard lamp 609 becomes extinguished.

Tubes62l to 626', inclusive, relays 63|' to 636', inclusive, lamps 64|' to 646', inclusive, relay 601', key 608 and lamp 609 function in a similar manner with respect to the oscillator circuits represented by the box 500 and with respect to the switching relays 5|| to 5|5, inclusive.

In order that the maintenance man may be informed of any failure of the commercial source 628 of alternating current which is provided for extinguishing tubes 62| to 626, inclusive, and 62| to 626', inclusive, upon the failure of their associated oscillator circuits, the source is inductively connected through another secondary winding of the transformer 621 and the rectifier bridge 649 over the front contacts of relay 650 with the winding of relay 629 whereby relay 629 is maintained operated. Should the source of power fail relay 629 will release thereby removing ground from the contacts of relays 63| to 636, inclusive, and 63|' to 636', inclusive, to prevent their release from operating either relay 601 or relay 601' and from lighting associated lamps 64| to 646, inclusive, and 64|' to 646', inclusive; to close the alarm circuits 641 il 648 and to close the circuit of guard lamp In View of the fact that the oscillator circuits which normally supply even numbered operators positions including the position fully disclosed in Figs. 2 and 3, have not been disclosed in detail, it will be assumed in order to clarify the following description of the operation of the circuits that the switching relays 5|| to 5|5, inclusive, have been operated and that the oscillator circuits of Fig. 4 are therefore supplying all of the operators positions.

It will be recalled that the operator has through the depression of key-pulsing key 306 caused the operation of relays 303, 301 and 3| 2. As soon as a sender has become connected to the link circuit and such connection has been signaled by the operation of sleeve relay 2 I9, a circuit is established from ground over its contacts, conductor 348, the No. 4 lower contacts of relay 303, the inner lower contacts of relay 301 to battery through the winding of relay 3|4. Relay 3|4 operates opening the locking circuit of relay 301 at its upper back contact and establishing a circuit from the right terminal of the secondary winding of transformer KP, over conductor 4|8, the upper back Contact of relay 505, conductor 5|8, the upper front contact of relay 5|5, conductor SIS, resistance 620, the upper front contact of relay 3|4, through the right windings of' repeating coil 3|?, the lower front Contact of relay 3|!|, resistance 32 i ,conductor SI5, the middle upper front contact of relay 515, conductor 5|9, the middle upper back contact of relay 505 and conductor 4|9 to the left terminal of the secondary winding of transformer KP. By reference to the preceding table, it will be noted that output circuits from the oscillator circuits 404 and 406, having frequencies of 1300 and 1700 cycles, are impressed upon the primary winding of transformer KP and thus current of these frequencies is impressed upon the right windings of repeating coil 3I1 over the circuit just traced.

peating coil '504, returning over trunk conductor H0, rings of jack '00 and plug 224, the No. 4 lower front contact of relay 203, conductor 201, the No. 2 lower front contact of relay 303 to the lower terminal of the secondary winding of repeating coil 3H. The current is further inductively transmitted over conductors '05 and '|00 of the interoflice trunk through the secondary winding of repeating coil '04 and the primary winding of repeating coil 'i133 and is inductively transmitted to the key-puising receiving circuit over a path extending from the upper terminal of the secondary winding of repeating coil 123, over the tip talking conductor extending through the link circuit 108, conductor 'HS through the padding resistances i000, |005 and |002, lter circuit |003 bridged by the padding resistance |005, through the primary winding or transformer |004, returning through the iilter circuit |003, through padding resistances E? and |003, over ring conductor 2'2. and the ring tall;- ing conductor extending through the link circuit ?03 to the lower terminal of the secondary Winding of repeating coil 103.

By the transformer i004 the signaling current is impressed through the windings oi transformer |000 upon the input circuit oi the primary ampiifier tube mit, the output of which is impressed upon the primary windings o transformer li. rIhe secondary windings of transformer |055 are coupled to the primary winding of transformer |0i2 and to the primary winding of transformer |053 whereby the signaling current is further impressed upon the input circuit of the secondary amplifier tube i014, upon the input circuit of the volume control tube |0I5 and upon the input circuit of the amplifier tube Il. The output of the secondary amplifier tube I0|4 is further applied through the transformer ll' and over conductor IOIS, thence in parallel through the tuned filters 00| to 000, inclusive, and the assoicated transformers SII to SIG, inclusive, upon the input circuits of the associated gas-illed tubes 02! to 20, inclusive. The filter circuits 90| to 000, inclusive, are so tuned as to pass frequencies of '700, 900, 1100, 1300 and 1700 cycles respectively. The tubes 02| to 92S, inclusive, are of the cold cathode gasfilled type and are normally so conditioned as to be non-conducting. The amplifier tubes |050, I0|4 and lll and the volume control tube |0|5 are vacuum tubes or" the pentode type and the tube |00 interposed between the secondary of transformer |0l3 and the input circuit of the volume control tube |055 is of the cold cathode gas-lled type.

Heater current is supplied to the filaments o tubes |0i0, Iili and IOIf`, in series, from the source |020 through the filament oi IOIo`, resistance I02I, the filaments of tubes i0|5 and I0|0 over normal contacts or test jack |022 to ground through resistance i023 and heater current is supplied to the filament of the tube I0l4 from battery through such lament, through resistances |024 and |025 over the normal contacts of test jack |025 and through resistance |021 to ground. The control grid of the primary am- -pliiier tube |0l0 is negatively biased from battery i020 through the secondary winding of transformer |000 from a point between resistances |020 and I 020 of the potential divider including battery |020. Negative bias is applied to the control grid of tube i055 from this potential divider through resistances i030 and |03! and negative bias is applied to the control grid of the secondary amplifier tube |0i4 from the battery supplying the lament circuit thereof through resistances |024 and i032. Anode potential is supplied to tube |0|0 from the 130 volt source 90'! of positive potential over conductor 00S, through the winding or alarm relay |033, normal contacts of test jack |004, impedance coil |035 and through the primary windings of transformer I0i| and to the anode of tube I0i5 in a branch of such circuit extending through resistance |030. Anode potential is applied from the source @0l over conductor 000 through the winding of alarm relay |03?, normal contacts of test jack |030 and impedance coil |030 to the anode of tube |00 and anode potential is applied from conductor 000 through the winding of alarm relay |00 over the normal contacts of test jack I04I and the primary winding of transformer I0| to the anode of tube iil.

The tubes mit, I0|4, Iii and |050 should normally conduct sunicient space current across the cathode-anode paths therethrough to maintain alarm relays |033, i031 and |040 operated. However, should one of the tubes fail and its associated anode-alarm relay become released the circuit for relay |042 is thereby closed and relay |042 upon operating establishes an alarm circuit extending from ground at the normal contacts of key |003, normal contacts of test jacks |022 and |020 and the right contacts of relay |042, establishes the circuit for guard lamp |045 and establishes a circuit from ground over its inner left contacts and conductor |044 to battery through the upper winding of make-busy relay I whereupon relay- 00| operates to render the receiving circuit busy to al1 link circuits. Noting the alarm the maintenance man operates key |043 to open the alarm circuit, to connect ground to hold make-busy relay 80| operated and to close another circuit of guard lamp |045 which remains lighted until the trouble condition has been removed, relay |042 has been released and the key |043 restored to normal.

1t having been assumed that a key-pulsing signal comprising current of the two frequencies 1300 and 1700 cycles has been transmitted, the amplifier tube |0I0 amplies such signaling current and impresses it upon the input circuits of amplier tubes I0|4 and IOI0 and across the control gap of the gas-lled tube |0|0. The tube I0! 9 together with pentode tube Ill, resistances |030, |040 and |041 and varistor |048 constitute a volume limiter of the type fully disclosed in Patent No. 2,208,752, granted July 23, 1940, to S. Doba, Jr. The varistor |040 is in the form of a copper-oxide bridge having two vertices connected across the transmission line between the transformers |004 and |009. The tubes |0|5 and |010 together with a resistance bridge three arms of which comprise the resistances |030, |045 and |041 and the fourth arm of which includes the cathode-anode path through the tube |0|5, control the varistor |040. The source of potential 001 is connected across two vertices of the resistance bridge and the other two vertices are connected to the free vertice-s of the varistor |048.

Under normal operating conditions with no energy peaks on the transmission line above a predetermined level, the resistance bridge is balanced so that no current is supplied therefrom to the varistor |048. Should a peak of energy above the predetermined level occur, the potential applied across the cathodes of the tube ||9 from the secondary winding of transformer IOI3 will cause this tube to ignite and to establish an operating circuit extending from the source of potential |020, thence as traced through resistance I02|, resistance |049 across the cathodeanode path of tube |0|9 through resistance |050 and delay condenser |05|, shunted by resistance |052 to ground. The condenser |05| charges in this circuit and when fully charged, the potential applied to the contr'ol grid |053 decreases the now of space current through said tube thereby unbalancing the resistance bridge and effecting current flow from battery 901 through the varistor |048. The impedance of the varistor is thus lowered to produce a shunting action on the transmission line.

The signaling current applied to the ampliiier tube |0|B causes a flow of alternating current therethrough over the circuit extending from ground across the cathode-anode path through the tube between the cathode |054 and the anode |055 over conductor |056, through condenser 909, the varistor 9|0 and resistance 951 to ground. This current is rectiiied by the varistor and applied through the left or operating winding of polarized relay 9|1. Normally the current applied through the right or biasing winding of relay 9|1 holds the armature of such relay against its back contact. When, however, current flows in its left winding due to an incoming signal, it overcomes the effect of the right winding to operate the relay armature into engagement with its front contact at which time resistance 951 becomes shunted by direct ground applied over the back contact of polarized relay 9|8 to increase the current flowing in the left winding of relay 9|1 to insure that such relay will continue to hold its armature against its front contact so long as the signal continues even though the signal strength should diminish. Relay 9|1 also opens the discharge path of condenser 9|9 through resistance 920 and over the back contact of relay 9|1 to ground at the back contact of relay 9|8 and opens the circuit extending from ground over the back Contact of relay 9|8, the back contact of relay 9|1 through resistances 920 and 921 and the left winding of polarized relay 928 to battery.

Prior to the opening of the circuit through the left winding of relay 928, this winding has been so energized as to hold the armature of relay 928 against its front contact. Following the disengagement of the armature of relay 9|1 from its back contact and the opening of the discharge path through condenser 9|9, current continues to ilow through the left winding of relay 928, resistance 921 and condenser 9|9 to charge such condenser and until the charging current ceases to flow, the armature of relay 928 is still held against its front contact by the energization of its left winding. When charging current ceases to flow through its left winding, the right or biasing winding of relay 928 becomes eiective to move its armature into engagement with its back contact to connect ground from the right back contact of relay 929, back contact of relay 928, left back Contact of relay 929 and conductor 930 to the cathodes of all gas-lled tubes 92| to 929, inclusive, whereby such tubes are rendered responsive to incoming signaling current. It will be noted that the interval required to charge the condenser 9|9 following the movement of the armature of relay 9|1 in response to the incoming signal has introduced a delay to permit any transient frequencies which may occur in adjacent filters upon the initial closure of the signaling circuit to disappear before the tubes are rendered responsive or are enabled.

With the tubes 92| to 926, inclusive, enabled, the signaling current as further amplied by the secondary ampliiier tube |0|4 is impressed upon the paralleled filters to 906, inclusive, and since the first signal received is a key-pulsing signal, comprising current of the two frequencies 1300 and 1700 cycles, such current is passed through the filters 904 and 906 and impressed through the transformers 9|4 and 9|6 across the cathodes of tubes 924 and 926 causing such tubes to become conducting. Current now ows in a circuit from ground on conductor 930, over the cathode-anode path through tube` 924, right Winding or' responsive relay 934, through resistance 944 to the source of potential 901 and in a second circuit from ground on conductor 930, over the cathode-anode path through tube 926, right winding of responsive relay 936, through resistance 949 to the source of potential 901. The responsive relays are normally biased to hold their armatures in engagement with their back contacts by their middle or biasing windings which are connected in a series circuit from battery through resistance 931 and through such windings to ground. When, however, the right windings of relays 934 and 936 become energized as previously described in response to the key-pulsing signal, these windings overcome the biasing windings thereby moving the armatures of such relays into engagement with their front contacts.

With relays 934 and 938 thus operated, a circuit is established from ground over the upper front Contact of relay 1|5, the inner upper front contact of relay 128, the back contact of relay 138, conductor 139, the back contact of relay 809, conductor 8|0, the front contact of relay 934, the No. l right back contact of relay 938, the iront contact of relay 936, the No. 4 right normal contacts of relay 938, the winding of relay 939, conductor 949, the back contact of relay 941 to battery previously connected to conductor 136 through the operation of relays 1| 5 and 128. Relay 939 thereupon operates, closing a circuit for relay 948 from battery on conductor 940 through its winding and the front contact of relay 939, whereupon relay 948 operates and locks to ground over its right front contact and under the control oi relay 941. Upon the termination of the key-pulsing signal, relays 934 and 938 release, in turn releasing relay 939, whereupon with relay 948 operated, a circuit is established from ground over the back contact of relay 939, the inner left front contact of relay 948 and the winding of relay 938 to battery on conductor 940 and a second circuit is established from ground on the back contact of relay 939, the outer left front contact of relay 948, through the winding of relay 929 to battery on conductor 940. Relay 938 upon operating connects the front contact of each of the relays 93| to 939, inclusive, to the left terminal of the left winding thereof, respectively, the other terminal of each of such windings of relays 93| to 935, inclusive, being connected to battery through the windingsof translator relays 803 to 891, respectively, and the other terminal of the left winding of relay 936 being connected to battery through the lower winding of relay SI5. Relay 929 upon operating removes ground at its right back contact from conductor 939- to disable the tubes 92| to 926, inclusive, and at its left front Contact prepares a circuit to include an additional timing interval controlled by relay 9|8. Upon the termination of the keypulsing signal relays 9H' and 9|9 release to their normal conditions as illustrated.

Normally when battery is connected to conductor |36 upon the seizure of the receiving circuit, the right winding of relay 941 is energized in a circuit extending from conductor |35 through resistance 949 and resistance 950 to ground thereby j holding its armature against its back contact. Since during the initiation of a call and before code impulses are transmtited, the transmission circuit may be briefly exposed to conversation, there may be p-resent current of the frequencies '700, 900, 1100 and 1500 cycles during the time that the key-pulsing signal is being transmitted and before relays 938 and 929 have become operated whereby one or more of the responsive relays 92|, 932, 923 and 935 may also be operated. Under this condition, a circuit is established from battery on conduct-or '|29 through resistance 95|, the left winding of relay 921 over one or more of the back contacts of relay 939 and the front contacts of one or more of relays 93|, 932 and 933, conductor 952 and the back contact of relay 8| I to ground on conductor 739 or over the frontcontact of relay 935, conductor 859 and over the back contact of relay 999 to ground on conductor 139. Relay 92's' will thereupon operate in about 5 milliseconds to disconnect battery from conductor 999- to arrest the operation of relays 938, 929, 948 and 929 or to release them depending upon their condition at the time relay 921 operates. As soon, however, as the extraneous frequencies disappear and only the key-pulsing code frequencies remain, any operated responsive relays such as 93 I, 932, 933 or 935 will release, thereby removing the shunting ground from condenser 953 whereupon condenser 953 will charge in a circuit from battery on conductor '|39 through resistance 95|, the left windnig of relay 991, condenser 953 and resistance 954 to ground. Since it requires condenser 953 about 50 milliseconds to charge, relay 921- does not release until this interval has expired. Following its release relays 939, 943, 93S and 929 function as previously described thereby conditioning the receiving circuit to receive code digit impulses. This feature thus serves as a guard to prevent a false registration due to voice currents incident to conversation.

Following the operation of relay 3M, it will be recalled that the holding circuit for relay 391 was opened whereby as soon as key 399 was released, relay 39'! released after an interval, in turn releasing relay 342 to discontinue the keypulsing signal. With relays 391 and 3|4 both released a circuit was established from ground over the lower contacts of relay 3&2, the lower back contact of relay 3M and the upper back contact of relay 39? to battery through lamp 369 which lighted as a signal to the operator that she could proceed with the keying of the desired line number on the key-set 3%9. It will be assumed that the number of the desired line is 2399 and that the operator therefore first momentarily operates the digit key 329 of her key-set. Current of the two frequencies 900 and 1300 cycles, as indicated by the preceding table, is thereupon impressed from the primary winding of transformer 2 upon the circuit which may be traced from the right terminal of the secondary winding of such transformer over conductor 420, the middle upper back contact of switching relay 994, conductor 529, middle upper front contact of switching relay 5M, conductor 322, resistance 329, the left contacts of the depressed key 328, over the inner upper back contact of relay 3M through the right windings of repeating coil 3H, over the inner lower back contact of relay 3M, the right contacts of key 328', resistance 325, conductor 32 3, the inner upper front contact of relay Sla, conductor 52|, the inner upper back contact of relay 592, and conductor 92| to the eft terminal of the secondary winding of transformer 2. In the manner previously described, this signaling current is transmittedy to the receiving circuit and is impressed upon the input circuit of tube |916 associated with the enabling circuit and is passed through the filters 992 and 99d tuned to the signaling frequencies `of 900 and 1300 cycles, respectively, and impressed upon the input circuits of tube 922 and 929.

In response to the impression of the signaling current on the input circuit of tube IMG, relay 9H- at once operates its armature to its frontl contact and after a delay interval, relay 928 operates its armature into engagement with its back contact, whereupon with relay 929 operated, a charging circuit is established for condenser 955 extending from battery through resistance 959 and the right winding of polarized relay 9i@ in parallel, left front contact of relay 929, back contact of relay 929 through condenser 955 to ground. The charging current through the right winding of relay 9&8 is of sufcient strength to quickly overcome the current iiowing through the left or biasing winding of such relay, whereupon it moves its armature from its back contact into engagement with its front contact thereby connecting enabling ground to conductor 939 and thus to the cathodes of tubes 92| to 926, inclusive, to render such tubes responsive to the incoming signaling current. Tubes 922 and 924 now respond to operate their associated responsive relays 932 and 939. As soon as condenser 955 becomes fully charged, current will cease to flow in the right winding of relay 9&8 and its left winding will then again become effective to move the armature into engagement with its back Contact thereby removing ground from conductor 939 to disable tubes 92| to 929, inclusive, thus the tubes are not enabled until after a predetermined interval following the receipt of the incoming signal to prevent their false operation by transient frequencies which may be present due to the establishment of the signaling circuit and are disabled after a further predetermined interval in the Aorder of 5 milliseconds or before the end of the signal to prevent false 0peration by transient frequencies which may be present at the time the signaling circuit is opened.

Responsive relay 932 upon operating establishes a locking circuit for itself from battery through the winding of translator relay 394 over conductor 862, the left winding of relay 932, the No. 2 left front contact of relay 938, the front Contact of relay 932 to ground on conductor 952 and relay 934 upon operating establishes a locking circuit for itself from battery through the winding of translator relay 909 over conductor 8|3, through the left winding `of relay 939, the No. 1 right front contact of relay 939, the front Contact of relay 934 to ground on conductor 8I0. Translator relays 804 and 808 operate in these locking circuits.

Following the seizure of the receiving circuit and the connection of ground to conductor 124 by the energization of relay 1|5 and until translator relays are operated in response to the first keyed digit, relay 8|4 is held operated in a circuit from battery through its winding, the No. 1 lower back contact of relay 804, the No. 4 upper back contact of relay 803, the No. 4 upper back contact of relay 805, the No. '4 upper back contact of relay 801 and the No. 3 upper back contact of relay 808 to ground on conductor 124 thereby holding open the circuit for relay 8|5. The circuit of relay 8I5 may be traced from battery through the upper winding of relay 8|5, and the upper back contact of relay 8|8 and the back contact of relay 8| 4 to ground on conductor 124.

As soon, however, as translator relays 804 and 808 operate in response to the keying of the first digit, assumed to be the digit 2, relay 8|4 releases but relay 8|8 immediately operates if but two of the translator relays have operated, ina circuit which may be traced from battery through its winding, the No. 1 upper front contact of relay 804, the No. 1 lower back contact of relay 803, the No. 1 lower back contact of relay 805, the No. 3 upper back contact of relay 801, and the No. 2 upper front contact of relay 808 to ground on conductor 124. Since relay 8|5 is slow to operate, it does not have suicient time to become operated between the release of relay 8|4 and the subsequent operation of relay 8|6. If, however, more than two of the translator relays should operate, then relay 8|8 would fail to operate and relay 8|5 would then operate and lock over its inner upper contact to ground on conductor 124 for a purpose to be later described. With translator relays 804 and 808 both operated and relay 8|8 operated, battery is connected over the upper front contact of relay 128, conductor 140, upper back contact of relay 8 I 5, inner upper front contact of relay 8|8, the No. 2 lower front Contact of relay 808, the No. 3 lower front contact of relay 804, through high resistance 8|1 and low resistance 8 I 8 in series, thence over conductor 13| to the sender and over the upper front contact of relay 8|8, the No. 1 upper front contact of relay 808, and the No. 2 lower front contact of relay 804 through low resistance 8|9 and conductor 131 to the sender. Relays in the sender respond to register the thousands digit 2 therein.

Relay 8| 8 upon operating also establishes a circuit from ground over its lower front contact, conductor 820 and through the upper normal contacts and winding of counting relay 14| to battery through resistance 142 whereupon relay 14| operates and locks over its upper alternate contacts to ground on conductor 124 and prepares a circuit from ground over such contacts through the winding of counting relay 143 and resistance 144 to battery but relay 143 does not operate at this time since its winding is shunted over its lower back contact so long as ground remains connected to conductor 820.

Prior to the operation of relay 8|8, the lower windings of polarized relays 809 and 8|| are energized in a circuit extending from battery over the upper normal contacts of jack 823 through the lower windings of such relays in parallel, thence over the lower normal contacts of jack 823 and the lower back contact of relay 8|8 to ground on conductor 124 and the upper windings of such relays are energized in a circuit extending from battery over the upper normal contacts of jack 824 through such upper windings in series over the lower normal contacts of jack 824 and through resistance 825 to ground on conductor 124 whereby such relays are energized in such a manner as to hold their armatures against their back contacts. When, however, relay 8I8 operates as previously described and opens its back contact, the circuit just traced through the lower windings of relays 809 and 8|| is opened and a charging circuit for condenser 82| is closed through such windings from battery as traced over the lower normal contacts of jack 823, thence through resistance 822 and condenser 82| to ground on conductor 124 whereby the charging current maintains the lower windings of relays 809 and 8|I energized until condenser 82| becomes fully charged. When the charging current ceases to flow through the lower windings of these relays, the upper windings thereof become effective to move the armatures thereof to open their back contacts whereupon ground is removed from conductors 8| 0 and 952 and responsive relays 932 and 934 and translator relays 804 and 808 release in turn releasing relay 8|8. The delay interval introduced by the charging time of condenser 82| thus insures that the translator relays 804 and 808 shall remain operated for a suicient interval to cause register relays in the sender to be operated and locked. When relay 8|8 releases, a discharge circuit is established over its lower back contact whereby condenser 82| becomes discharged and relays 809 and 8|| again cause their armatures to engage with their back contacts. Relay 8|8 also disconnects ground from conductor 820 whereupon counting relay 143 operates. Thus at the completion of the registration of the rst or thousands digit, both counting relays 14| and 143 are operated and with all of the translator relays 803 to 801, inclusive, released, relay 8| 4 is again operated over the circuit previously traced.

The operator will next momentarily depress the digit key 329 to register the digit 3 whereupon current of the two frequencies 1300 and 1500 cycles, as indicated in the preceding table, will be impressed from the primary winding of transformer 3 upon the circuit which may be traced from the right terminal of the secondary winding of such transformer over conductor 422, the inner lower back contact of switching relay 504, conductor 522, the inner lower front Contact of switching relay 5|4, conductor 328, resistance 321, the left contacts of key 329, the inner upper back contact of relay 3|4, the right windings of repeating coil 3|1, the inner lower back contact of relay 3|4, the right contacts of key 329, resistance 330, conductor 33|, the lower front contact of relay 5I4, conductor 523, the lower back contact of relay 504 and conductor 423 to the left terminal of the secondary winding of transformer 3. In the manner previously described the signaling current is transmitted to the receiving circuit and causes the functioning of the enabler circuit thereat and is impressed through the filters 904 and 905 upon the input circuits of tubes 924 and 925 which upon being enabled by the enabler circuit cause the operation of the associated responsive relays 934 and 935. Relay 934 upon operating closes a locking circuit for itself extending from battery through the winding of translator relay 808 over conductor 8 3, the left winding of relay 934, the No. 1 right front contact of relay 938, the front Contact of relay 

